The present invention is related to turbomachinery and more particularly, to a spacer apparatus for retaining a compressor rotor blade while improving gas turbine efficiency. Gas turbine engines include a compressor having a compressor rotor comprising a plurality of compressor stages. Air flowing into the compressor is compressed at each stage to such an extent that the pressure after the last few stages can reach approximately seventeen times the atmospheric pressure. Each compressor stage comprises a plurality of rotor blades or buckets mounted to the periphery or rim of a rotor wheel in a spaced relationship between adjacent blades. This spaced relationship is conventionally maintained by spacer blocks mounted on opposing sides of the rotor blade mounted on the rim. A typical compressor rotor can have dozen of rotor blades and multiple dozens of spacer blocks.
Manufacturers of advanced gas turbine engines seek to design and develop engines with increased reliability and reduced life cycle cost. Life cycle cost for users of gas turbine engines is directly related to its efficiency. The life cycle cost can be related to many factors such as, the initial construction, fuel consumption, periodic maintenance, and other cost factors incurred during the life of a gas turbine engine. Thus, any improvement that can reduce life cycle costs is a valuable one. A primary way of reducing life cycle cost is to improve the efficiency of a gas turbine engine, thereby reducing fuel consumption. Another way to reduce life cycle cost is to maximize gas turbine availability via increased reliability, which in turn reduces the maintenance overhaul costs.
Of those factors affecting increases in life cycle cost, the overall airfoil response is important to consider. There are a number of ways to increase airfoil efficiencies. Engine designers can impose small changes to the airfoil leading edge and trailing edge angles, optimize distances between airfoils, impose airfoil leans, and impose airfoil sweep. Another way to increase airfoil operability or efficiencies is to minimize leakages around the airfoil tip and/or dovetail. These leakages or gaps are defined as the distance between the rotor blade tip and the compressor case or machined gaps under the dovetail in relation to rotor wheel rim. Relatively large changes in the radial clearance of the tip or machined dovetail gap in combination with high compression of the air, can prematurely force air to leak around these blade features. Most gas turbine engines are designed with a tolerance level or margin to account for some leakage. In general, an excessively large radial gap lowers the efficiency by increasing air leakage reducing engine performance and increasing fuel consumption. Therefore, it is desirable to minimize these leakages for the engine.
Dovetail slot leakage is a prime factor that affects the engine performance. It has been determined that the amount of machine gaps around and under the airfoil dovetails is far in excess of those gaps existing between the rotor blade tip and the compressor case. When viewed from an engine system standpoint, each compressor stage has dozens of leakage pathways formed between the abutting or interfacing parts of the rotor blade, slot, and spacer blocks. Each engine has dozens of compressor stages that allow these so called cumulative leaks to increase. The result of these leaks is that several tenths of the percent flow are lost and thus fuel consumption is increased. In addition, reducing slot leakage improves the ability to better control the radial tip clearance of the airfoil relative to the compressor case walls.
Manufacturing a conventional compressor rotor can involve numerous man-hours and equipment operation time. Conventionally, the compressor rotor is assembled by stacking compressor stages, one by one, within an axial relationship to each other. A typical compressor stage can weight almost 10,000 pounds. A conventional technique for securing the rotor blades to the rotor wheel is to form flat bottom slot in the wheel rim having a cross-section matching the blade dovetail flat bottom shape. Each blade has a dovetail portion formed with a complementary dovetail feature that interlocks with the dovetail region of the rim to secure the blade to the rotor.
Most blade dovetails in industrial gas turbines are secured in a process call xe2x80x9cstakingxe2x80x9d. The assembly process requires the placing of a first dovetail spacer in the wheel rim slot, then placing a rotor blade in the slot of the rim and finally placing a second spacer. These three pieces (e.g., first spacer, rotor blade, and second spacer) are then staked in place by deforming metal material around the blade dovetail with a tool similar to a nail punch. Each of the hundreds of rotor blades and spacer blocks must be manually assembled into the slot of the rotor wheel rim. This process is repeated for each compressor stage, each compressor engine and any repairs that might occur over the life of the engine. This assembly process is time consuming and costly.
Compressor rotors undergo a periodic inspection and overhaul process after a predetermined of number of operating hours. In the inspection and overhaul process, the rotor blades and spacers blocks are removed from the rotor wheel, i.e. the original xe2x80x9cstakesxe2x80x9d are ground out. This removal and re-staking process can be time consuming and costly. Thus, the cost of constructing a gas turbine and of performing periodic inspections can be reduced, if steps in the assembly process are eliminated.
In seeking to reduce life cycle costs, the cost of retooling and reconfiguring manufacturing operations should be avoided. Due to increased life expectations, rotor wheel designs have been migrating towards a slot in the rim having a rounded bottom geometry instead of a flat bottom geometry. This evolution necessitates retooling equipment and redesigning the dovetail shape of rotor blades in a rounded design. This retooling, both for the slots in the wheel rims and the blade dovetails can be a very costly process and includes such items as reprogramming equipment, designing new manufacturing fixtures, additional compressor stage stacking operations, and discarding the flat bottom dovetail rotor blades. Thus it is desirable to reduce or eliminate, the discarding of flat bottom dovetail blades, and the associated costs of using rounded bottom dovetail blades.
Briefly, the present invention fulfills the need to reduce the life cycle cost by improving efficiency and reducing maintenance cost in a gas turbine engine. Broadly, in one aspect of the invention, a rotor blade spacer apparatus for a compressor comprises a body having a blade retaining portion, and a spacer portion, in which the body includes a dovetail portion for mating engagement with a slot on a compressor rotor wheel. The spacer apparatus fits within the slot of the rim of the rotor wheel to maintain a spaced relationship between adjacent rotor blades and, simultaneously, to retain and hold the dovetail portion of the rotor blade mounted within the spacer apparatus.
In another aspect of the invention, a spacer cartridge enables a rotor blade with a flat or semi-flat bottom geometry dovetail portion to be engaged within a round bottom dovetail slot of the rotor wheel enabling older flat bottom blades to be used in newer rotors. Enabling the use of existing flat bottom blades in rotors with rounded bottom slots provides significant cost savings. The dovetail portion of the rotor blade is retained in a pocket or notched portion of the spacer cartridge. The pocket is configured to retain the entire dovetail portion of the rotor blade. When the spacer cartridge is in use, surfaces of the dovetail portion abut the mating corresponding faces or surfaces of the pocket. The longitudinal face or surface on either side of the dovetail portion bears against a portion of the inner surfaces of the slot in the rim of the rotor wheel.
In one aspect, the present invention reduces the life cycle cost by improving the efficiency of a gas turbine engine because the spacer cartridge provides improved blade tip clearance control by reducing slot leakage in the compressor stage. The small radial clearance between the rotor blade tips and the compressor shroud remains more consistent during operation of the compressor rotor. Thus, less leakage margin is required and the fuel consumption is improved by using the rotor blade spacer apparatus.
In another aspect, the present invention also reduces life cycle cost by reducing the assembly time of the blades to the rotor rim. The spacer cartridge eliminates the assembly of two separate spacers block each of which had to be individually assembled in the rotor wheel rim. In addition, the spacer cartridge reduces waste of material by accurately holding the tip of a rotor blade outward for a tip grinding operation after the blades are on the rotor wheel. Further, the spacer cartridge increases the efficiency of operations and reduces overhead operating costs by reducing the number engineering drawings, and requiring warehousing and inventory control of fewer parts.